The present invention relates in general to transducers which sense and signal the values of changeable physical parameters such as pressure, temperature, humidity, speed, flow rates and the like. More particularly, the invention pertains to the compensation for, or the elimination of, inaccuracies in the signaled value of a changeable physical parameter due to offset or drift exhibited by a transducer as a result of variations in conditions to which it is subjected.
In many and diverse control systems, a given physical variable is controlled by sensing its value, comparing it with a set point, and energizing some correcting device so as to reduce or eliminate the difference between the desired and the actual values. In some cases the variable is directly sensed and directly corrected; for example the temperature of a room may be sensed and compared with a set point by a thermostat which produces an output signal proportional to the difference or error--and such output signal is utilized directly or indirectly to adjust the rate of fuel input to a furnace burner. In other instances, the corrective action is determined as a function of several changeable factors; for example, the temperature of a room might be correctively restored to and held at a set point by some actuator (such as a motorized fuel valve for a heat-supplying furnace) moved as a combined function of sensed room temperature, set-point temperature, outside air temperature, humidity, and the velocity or volumetric flow rate at which heated air is blown into the room.
In all such diverse control systems, there is a need to sense and signal the value of one or more changeable physical parameters. The word "parameter" should be taken here according to common usage rather than according to a formal dictionary meaning. As used in this application, the term "changeable physical parameter" is to have a generic meaning which designates a physical condition which changes or varies either in an unforeseen or in a generally predictable fashion; it applies to such physical variables as temperature, humidity, linear velocity, rotational speed, pressure in a liquid or gas, flow velocity, volumetric flow rate, chemical concentration, and even the position of a movable member.
It is the province of a "transducer" to sense a changeable physical parameter and produce some type of signal which changes according to a known function with changes in the value of the parameter. The changes in signal magnitude are related, linearly or non-linearly, to the changes in the parameter by a transfer function or scale factor. For example, a d.c. tachometer might be characterized as producing a signal voltage with a transfer function factor of 0.1 volt per r.p.m.; the sensed parameter is rotational speed, but the output voltage signal uniquely relates to and designates the value of the speed at any given time.
It is inevitable, however, that transducers will be subjected to changeable conditions, mostly environmental, other than the sensed parameter. Changes in such other conditions can cause significant, undesired changes in the transducer's output signal, thereby creating inaccuracy in the signaled value of the parameter due to offset or drift. For example, an air filled pressure-sensing bellows coupled to shift the wiper of a potentiometer (and thereby change a signal voltage from the potentiometer) will expand or contract not only as the pressure of the surrounding medium changes but also spuriously as the temperature of that medium changes. Although some transducers are essentially immune from offset changes when the associated physical conditions vary unforeseeably, others exhibit wide changes in their output signals as temperature, humidity, aging, wear, pressure or some factor (other than the sensed parameter) changes. When a very low range (not departing widely from zero) of the sensed parameter is to be signaled, and the gain or transfer function of the transducer per se is relatively low, then changes in offset due to variable conditions may be so great as to dwarf or mask the signal variation resulting from changes in the sensed parameter.
For example, applicants were faced with a specific need to sense and signal the value of a changeable differential pressure created in a Pitot tube used to determine the velocity of flowing air. The differential pressure would vary only from about 0 to 1.5 inches of water column (i.e., from about 0 to 0.054 p.s.i.). One particular type of pressure transducer (namely, a piezoresistive Wheatstone bridge formed by integrated circuit techniques on a silicon chip diaphragm) was desireable for certain characteristics (e.g., small size) but it exhibited large and nonlinear offsets in its output signal with changes in temperature. The offsets of such a silicon transducer are small in relation to the full scale output voltage change if pressure are sensed over a range from 0 to 5 p.s.i., and thus may be reasonably compensated by known use of counteractive temperature sensitive resistors or diodes. But when a full scale range of 0 to 0.54 p.s.i. is to be signaled, the temperature offset in the output voltage may be four or five times the full scale change in output voltage due to changes in sensed pressure. Unless the effects of temperature offset are essentially eliminated, output voltage from such a transducer used over a small pressure range would be useless because the output signal would lack sufficient accuracy and resolution.